The metal to metal seal formed in a valve between a valve closure member and a valve seat determines the accuracy at which the fluid flowing through the valve is controlled. Leakage results when the surfaces between the valve closure and the valve seat do not mate correctly. This leakage is detrimental in systems where precise flow control is desired. Similarly, the amount of gasoline leakage from a fuel injector has an effect on evaporative emissions. Government legislation has reduced the amount of automotive evaporative emissions so customers are requiring more stringent fuel injector leakage.
A valve seat is typically a ground hardened conical seat (Rc>55). The valve closure member is also of a similar material and hardness. This conical valve seat and valve closure member must have low roundness in order to produce a tight seal to prevent leakage. One method used to produce low seat roundness resulting in a tight seal between the closure member and the valve seat is grinding. Grinding greatly influences the accuracy and reliability of the fluid valve, however, the roundness tolerances for low leakage rates are in sub micron range. As a result, grinding becomes an extremely expensive manufacturing procedure. Such activities will increase manufacturing costs and therefore there exists a need for alternate procedures that are less costly and desirable.
Another method for manufacturing a closure member and valve seat applies an axial compressive load to force the closure member against the seat, coining the closure member to the seat. The method described in U.S. Pat. No. 5,081,766 produces a valve assembly that is capable of accurate and reliable fluid metering yet avoids expensive tolerance control on surface finishing and part dimensioning. The method disclosed by this patent involves the inclusion of an additional step in the assembly process, a coining step, but eliminates the necessity for stricter tolerances on surface finish and part dimensioning. Accordingly, reconfiguration of existing manufacturing equipment and processes requires merely adding the coining step to reduced leakage through the injector. This coining step however does not involve the use of a coining die to coin the part. Rather the coining step involves the application of axial compressive load to force a rounded distal end of the closure member against a conical surface of the seat so that the coining action occurs as an annular zone of surface contact between the closure member and the seat. The force of application is preferably conducted in a particular manner so that the closure member is neither irreversibly bent or buckled by the coining step. This step is conducted during the assembly process so that neither the solenoid nor the spring which are the operating mechanism in the completed injector has an influence on the result of coining.
It would be beneficial to develop a method and apparatus to form a better seal between the closure member and the seat using part materials and initial geometry configuration when a closure member first contacts valve seat during assembly of the fuel injector to assure improved seal and manufacturing cost savings.